Relief structures, such as printed or stamped diffractive optical surface structures and similar security features, are exposed to counterfeiting by mechanical lifting when the structure is not coated and therefore exposed. The coating required to protect such structures must be of a sufficiently high refractive index compared to the substrate to which the feature is applied to maintain visibility of the feature. Such high refractive index (HRI) coatings, used for relief structures applied to a variety of substrates, should preserve all or as much of the original, intended colour, clarity and visibility of the non-coated feature as possible. Therefore, high transparency and low (or no) colour are important requirements if a diffractive effect, which is visible both in reflection and transmission, is to be achieved, especially for relief structures applied to clear substrates. To date, these requirements have been difficult to fulfil by a printed coating where good adhesion, chemical resistance and high durability are required, whilst also rendering the relief structure resistant to mechanical lifting.
Traditionally, HRI coatings based on titanium dioxide (titania) or zinc sulfide or zinc selenide have been deposited onto a surface by vacuum deposition, a technique which requires high temperatures and is expensive. Other sputtering techniques of metal layer deposition may not be easily adapted to high speed printing. Furthermore, some surface structures require a layer of high refractive index coating that is of the order of microns thick. Metallized coatings are unsuitable, as thicker coatings are expensive and can impart a highly reflective surface with noticeable color in the coating. Thicker metalized coatings are also less robust. High refractive index coatings made of metal- containing polymer are known, but utilization requires coating and subsequent curing at high temperature to provide a metallic coating on the substrate (Wang et al, Proceedings of SPIE, vol. 5724, 2005). This technique requires a processing temperature exceeding 100° C. and often up to 200° C., which is too high for use with a polymer substrate, and is not amenable to reel-to-reel printing processes.
Metal dioxide particles of various sizes employed in suspension or other liquid formulations are known in HRI coating technology mainly for application in electronic displays. Clarity of the coating can be improved by reducing the coat weight. However this can reduce the effectiveness of protecting the structure on certain relief structures. Metal dioxide nanopowders, in which the particles are coated with various functional groups, may be dispersed in a suitable solvent and utilized as a coating formulation. However, these materials can suffer from poor adhesion to polymer substrates in the absence of suitable additives.
The conventional method of utilizing high refractive metal oxides is as an additive, mixed with a resin or carrier. This may improve adhesion; however, it also leads to a reduction in refractive index, depending on the ratio of metal oxide to resin employed. On embossed substrates, such formulations based on titanium dioxide alone yield coatings with very poor adhesion and low transparency. Adhesion can be improved by increasing the amount of resin, but refractive index is then reduced.
Whilst high refractive index polymers containing halogens are available, they are undesirable from cost and environmental standpoints.
There is therefore a need for a high refractive index coating that provides a durable coating which is resistant to mechanical lifting, transparent, of high adhesion, whilst at the same time being simple to employ.